1. Field of the Invention
The present invention relates to a rear-focus-type zoom lens equipped with an index-distribution-type lens and, in particular, to a high-aperture-ratio, high-zoom-ratio zoom lens which has a zoom ratio of approximately 6, an f-number of approximately 1.8 and is suitable for use in a photographic camera, a video camera, a broadcasting camera or the like.
2. Description of the Related Art
Regarding the zoom lens of a photographic camera, a video camera or the like, various kinds of so-called rear-focus-type zoom lenses have been proposed up to the present time, in which focusing is effected by moving a lens group other than a first lens group which is on the object side.
Generally speaking, the effective diameter of the first lens group of a rear-focus-type zoom lens is smaller than that of a zoom lens of the type in which focusing is effected by moving the first lens group, thus reducing the size of the entire lens system. Further, the rear-focus-type zoom lens is advantageous in that it facilitates close focus shots, and in particular, ultra-close focus shots. In addition, the rear-focus-type zoom lens requires less driving power than other types of zoom lenses and therefore makes it possible to perform focusing quickly. Due to these advantages, zoom lenses of this type are being widely used as the zoom lenses for video cameras, which are required to be reduced in size and have an increase in zoom ratio.
One example of a rear focus-type zoom lens is shown in Japanese Patent Laid-Open No. 62-247316, according to which there are arranged, in order from the object side, a first lens group having a positive refracting power, a second lens group having a negative refracting power, a third lens group having a positive refracting power, and a fourth lens group having a positive refracting power. In this example, the second lens group is moved so as to effect zooming, and the fourth lens group is moved so as to compensate for an image plane fluctuation which accompanies the zooming, and also to perform focusing.
To ensure a fixed level of zoom ratio with a zoom lens having a reduced size, it is generally necessary to choose of the lens groups.
However, an increase in the refracting powers of the lens groups will cause variations in aberration, which will accompany the zooming step. To compensate for this, it is necessary to increase the number of lenses in each lens group, with the result that the thickness in the optical-axis direction of each lens group increases. Accordingly, the distance between the principal points has to be increased, resulting in an increase in the length of the entire zoom lens. In view of this, a proposal has been made in Japanese Patent Laid-Open No. 2-39011, according to which the zoom lens adopts, in addition to a spherical lens surface configuration, an aspherical lens surface configuration, thereby making it possible to ensure a fixed zooming ratio while attaining a reduction in the size of the entire lens system.
Generally speaking, by adopting the rear-focus system, it is possible to reduce the size of the entire lens system and perform focusing quickly, and facilitates close focus shots, as stated above.
In addition, by utilizing the aspherical lens surface configuration, it is possible to make the length of the entire zoom lens relatively small while maintaining a satisfactory optical performance.
However, the aspherical lens surface configuration does not provide such optical advantages as, for example, a reduction in the Petzval sum. Therefore, a lens group which is required to be achromatic must be equipped with two lenses which are different at least in dispersion characteristics so as to compensate for a chromatic aberration within the lens group. Further, it is also necessary to set the refracting power of the lens group within a range where the Petzval sum is not excessive.
Thus, even with a lens having an aspherical lens surface configuration, there has been a limit to how much the size of the entire lens system can be reduced while retaining a fixed zoom ratio.
There has also been proposed, in Japanese Patent Laid-Open No. 61-172110, a zoom lens of a so-called index distribution type, which uses a glass exhibiting a continuously changing refractive index distribution.